Image projection is one of the major fields of application for visible light generated by a high intensity discharge (“HID”) lamp. The conventional HID lamp optimized for visible light has major attributes that render it particularly suitable for use in image projection. Such HID lamp typically emits light from a plasma arc formed inside an envelope between two electrodes which are spaced a particular distance apart. The radiation spectrum of the light emitted from the HID lamp depends on the gases and other materials contained within the lamp (the “fill”). In a conventional projection system, the light from the lamp is collected via a series of optical elements and projected through an image gate onto a screen to form a projected image. The element which forms the image at the image gate can be film or any type of a light modulator, e.g., liquid crystal displays (“LCD”), digital micro-mirror devices (“DMD”) or liquid crystal on silicon displays (“LCoS”). In image projection applications, the utility of the HID lamp may be defined by its optical efficiency, power efficiency, color rendition, arc stability (absence of “flicker”), arc gap, physical size, initial cost, operating cost, and overall system cost. HID lamps can also be designed to produce ultraviolet (“UV”) or infra-red (“IR”) radiation for applications with similar performance requirements.
A conventional HID lamp presently has light transmissive envelopes made from quartz or polycrystalline alumina (“PCA”, also known as “ceramic” envelopes). In general, image projection applications require the HID lamp with a clear envelope, small arc sizes and narrow light beams. The HID lamp with quartz envelopes generally meets these requirements, however, PCA envelopes are translucent and generally not suitable for image projection and similar applications. The PCA envelope lamp is usually constructed with relatively large gaps as necessary for large light source applications. More recently, the HID lamp envelope has been made from poly-crystalline sapphire (“PCS”) which is produced by conversion in place of PCA envelopes. Although PCS envelopes improve light transmissivity and other characteristics of the envelope compared to PCA envelopes, PCS envelopes still have microscopic surface undulations that render them not suitable for most image display projection and related applications. Therefore, the conventional HID lamp continues to rely primarily on quartz envelopes.
The use of a quartz envelope places substantial limits on the conventional HID lamp in terms of meeting the above listed desired features for image projection. For example, the quartz envelope has a relatively low melting temperature, power load factor, thermal conductivity and tensile strength. Such considerations effect the lamp optical efficiency, efficacy, power capacity, size, life and the ability to control flicker. Furthermore, the quartz envelope is permeable to a number of additives, such as sodium or hydrogen, which are important in the spectral tailoring of the emitted light.
The Image Projection Industry has established that a correlated color temperature (“CCT”) of 6,500° K. (“D65 standard”) is the light source spectrum most desirable for image projection because it has a high color rendition index and is close to daylight quality. The conventional quartz envelope HID lamp is generally designed to operate at pressures from about 120 up to a maximum around 200 atmospheres utilizing a fill of pure mercury. However, a high pressure mercury lamp has CCT about 7,000° K. to 9,000° K. The light from such HID lamp must be filtered in order to achieve a more compatible CCT however filtering can reduce lamp efficiency by about 30 to 40%. Metal halide additives have typically been added to mercury lamps for the purpose of tailoring the light spectrum to a more desirable CCT (“metal halide” lamps). However, the effectiveness of metal halides is reduced as operating pressure increases to the point of minimal contribution at the maximum current operating pressures for the quartz envelope lamp. A conventional Image projection system uses light sources with a wide range of CCT from a typical 3,000° to 3,300° K. tungsten halogen lamps, to 4,000° to 5,000° K. for metal halide HID lamps, 5,500° to 6,500° K. for short arc Xenon lamps, and over 7,000° K. for a mercury lamp.
In the image projection field, the industry has moved steadily in recent years toward utilizing smaller light modulators based upon foundry fabricated silicon wafers, e.g., DMD and LCoS, with diagonals of 0.9 down to 0.5 inches. Such small apertures require that the HID lamp used have arc gaps in the range between 0.8 mm-1.3 mm in order to obtain an efficient optical match between the light emitted by the HID lamp and the aperture optics. As lamp gaps become smaller the efficacy of the HID lamp is reduced and the power that can be supplied to the plasma arc is limited by the envelope material thermal characteristics. In order to increase the efficacy of smaller arc gap lamps, the operating pressure must be increased. However, quartz envelope properties limit the pressure and power load factor that one can use in such HID lamps to about 200 atm and about 20 watts/cm2. Also, in applications such as image projection, lamps must be essentially flicker free. Flicker in an arc lamp is associated parametrically to the lamp bulb size and the fill pressure. Using conventional quartz envelopes, one needs to remain below 200 atm in lamp pressure in order to achieve flicker free operation.